This invention relates to filled polymers, particularly to a polybenzimidazole ("PBI") filled with glass bubbles or balloons.
Polymers are used to make a multitude of products, including a wide variety of molded articles. These molded articles often must have specific attributes that may not be provided by unfilled polymers. In such cases, it may be desirable to use polymers filled with materials such as graphite powder, chopped glass, mica flakes, carbon fibers, glass fibers, metallic powders, or other fillers. The filler may advantageously alter the mechanical or thermal properties of the polymer. Filled polymers also may be used to reduce costs, or for other reasons.
A variety of filled polymers are known in the art. For example, polyesters may be filled with talc or hollow glass spheres (i.e., glass bubbles) to improve thermal or mechanical properties, thermoplastics may be filled with metallic powders to produce thermally or electrically conductive articles, and ground silica may be used in polyurethanes to form potting compounds having favorable dielectric properties. See: Encyclopedia of Polymer Science and Engineering, Mark, Bikales, Overberger, & Menges, Eds., 2d Ed., Vol. 7, pp. 53-73, John Wiley & Sons, Inc., 1987.
Generally, the polymer to be filled is one that is easily melt processible so that it will readily flow around the filler particles during molding to produce an article having a relatively uniform composition. The ability of such polymers to completely surround the filler material minimizes potentially detrimental voids and heterogenous areas within the article.
Thermally intractable or infusible polymers such as PBI are not amenable to melt processing with filler materials. Although these polymers generally may be hot-pressed or cold-pressed and sintered by compression molding techniques, it is difficult to use these polymers to make filled molded articles because the polymer does not melt and flow around the filler particles.
Hollow glass bubbles, sometimes called glass spheres or microspheres, or glass balloons, may be made in a variety of sizes and densities. The outer diameter of the bubble, and the thickness of the glass bubble wall, determines the volume of air (or vacuum) inside the bubble, and therefore its density. The air (or vacuum) inside the hollow glass bubble also reduces the thermal conductivity of the glass. Glass bubbles of various densities (e.g., 0.2-0.6 g/cm.sup.3) and sizes (e.g., about 30-50 microns in diameter) are commercially available (e.g., SCOTCHLITE.TM. glass bubbles from 3M Corporation, having its headquarters in Minnesota).